Frank, K0BRA, published an article describing the AMRAD low frequency
upconverter in the April, 2002 issue of
QST Magazine. That project includes
a low pass filter that's designed to knock signals above 500kHz down to
size. The filter has two components: a seven-pole low pass filter, and
three sections of tunable notch filters that can be used on particularly
strong signals.

Cleverly, Frank designed the project so the LPF is on a separate
circuit board and you don't have to build the whole converter to take
advantage of the LPF. FAR
Circuits has PC boards for the upconverter available, and was willing
to sell me just the LPF board for $4.00 plus shipping.

I've built and installed the low pass filter, and it has solved the
problem my Austron LORAN-C receiver had with overload from the local AM
stations. This page shows the results I obtained when I measured the
filter's performance with my spectrum analyzer and tracking generator.

Low Pass Filter Only

I initially built and tested just the low pass filter section, and here are
the results.

This is the frequency response from 0 to 2MHz. The maximum
attenuation is a bit more than 60dB. Above 10MHz, this rolls off a bit
but stays better than 50dB through 40MHz. (Note: these images were
captured with the filter temporarily mounted. The final mount will have
better grounding and this may improve the stopband attenuation somewhat.)

This is the insertion loss shown at 1dB/division from 10Hz through 500kHz.

LPF Plus Notch Filters

Next, I added the three notch filter sections to the filter. I had to
experiment with the capacitance values to put the notches where I needed
them; the tuning range of the variable inductors is only a couple of hundred
kilohertz.

I started with the L3/C3 notch, which resonates nicely at 980kHz
with a 0.00027uF capacitor. Here are a couple of images of the LPF plus
L3/C3 filter response:

Note that these shots were taken with the filter outside its box. The
nominal input level was 0dBm. Also note the anti-resonance at about
1005kHz. Fortunately there aren't any local stations right at that spot.

I wired the L2/C2 notch to resonate at 1290kHz, the middle (and weakest) of
the three major signals. To get there, I used a 0.0027uF cap in series
with a 0.0039uF.

The L1/C1 filter is a parallel resonant trap, with a smaller inductor. I
configured it for the 1410kHz signal, which is the strongest here but is
also the furthest down the response curve of the LPF. To get there, I
used a 0.015uF cap.

I could tune L1/C1 to put the 1410kHz signal over 100dB down, but rather
than doing that I adjusted it to smooth out the peak above the 1290kHz
notch. I figured that 1410 was already far enough down and it didn't need
the additional notch.

Results

After installing all three notch filters and packaging the thing up in
a Hammond diecast box, here are the final results.

This is the full 0 to 40MHz sweep.

This is the more interesting 0 to 2Mhz range.

This is the insertion loss from 0 to 500kHz.

Note that the insertion loss is greater than in the LPF-only traces, and
is around 3dB. This is because I've now added the series input resistor
(33 ohms) that I didn't include earlier. This resistor provides a better
match to the source for out-of-band energy and thereby helps reduce
possible IMD problems.

As a measure of how much this filter helps in my situation, the AMRAD
active LF antenna delivers about -6dBm of (daytime) power at the output of
its two-way splitter (that means the actual single-port power level is
3dB above that). With the filter inserted, the power level drops to
about -38dBm, which ought to make any receiver quite a bit happier.